12 November 2013
Fire setting at Stone Age Norwegian quarries
Extraction marks in the Melsvik Stone Age chert quarries near Alta in northern Norway are difficult to explain by any other ancient technique than fire setting. University Museum of Tromso experimented with this important method.
Small, controlled 'bonfires' are enough to create shear stress and cracking. High temperatures greatly reduce the quality of the chert for tool making.
The quarries are the most important discovered in northern Norway so far, and may date to the so-called 'pioneer phase' around 9500 BCE, not long after the last ice sheet retreated. The quarries were particularly in use in the Early-Middle Mesolithic (7000 to 8000 BCE), providing material for knives, arrowheads, scrapers and so on.
The chert deposit is situated on a small hill, forming layers usually less than half a metre thick above and within Precambrian dolomite. This peculiar geology, where chert is often 'draped' around dissolving dolomite (or 'karst'), often leaves hollows between the two. This can be tested by banging the chert with big stones - the feeling is that they 'come back' to you, and breaking bigger pieces loose with such a brutal technique is out of the question. Chert is extremely hard, compact and tough, making it almost impossible to extract by using hammer-stones, and bone and stone wedges. On repeated blows what you usually get is pulverised stone unsuitable for tool making. Stone Age man had to employ more efficient techniques.
Looking at the level or slightly sloping excavated quarry faces there are probably 50 or more round, shallow depressions measuring about 0.5 to 1 metre across. They are usually convex at the bottom, a strong indicator that fire was used in their creation. In addition, beside and below zones of such depressions are thick layers of broken-up chert, all with sharp edges but without traces of man-made working. Such layers ought to represent the waste from fire setting.
The natural colour of the chert ranges from shiny greyish white to bluish and purplish. It becomes dull white upon exposure to fire above about 300 to 400 degrees centigrade - a result of micro-cracking, which alters the light reflection properties.
Researchers set out to replicate the method. They used birch wood for burning, though pine and mountain ash were also available in the Mesolithic. Hammer-stones, stone wedges and fire-hardened bone and antler wedges all in different sizes aided in removal of cracked stone. Five experiments were made.
Each fire had a diameter of about half a metre. Two or more fires beside each other worked best where the rock was under significant natural tension, creating shear stress in the area between and keeping the temperature low. It took 5 to 10 minutes to start the chert breaking at the surface, and 45 to 60 minutes to create deep, lateral cracks more than 15 centimetres below.
The temperature beneath the centre of the fire reached 400 to 500 degrees centigrade after 20 to 30 minutes. After 60 minutes it was in the range of 40 to 60 degrees at depths of 15 to 20 centimetres. After the initial surface cracking, the chert typically developed parallel, lateral cracks further down, breaking loose flakes with a thickness of 3 to 5 centimetres weighing from a few hundred grams to a kilo or more. In one experiment it was possible to break loose a block weighing 20 to 30 kilograms, with a maximum thickness of about 15 centimetres.
In cases where the rock was under significant natural tension, cracking sounded like making popcorn, with small chert fragments jumping up to two metres into the air. Formation of deeper cracks was followed by lengthy 'krrrrks' as tension was released.
Removal of flakes and blocks was undertaken using stone and bone or antler tools, and worked excellently, with a little hammering here and there and simple wedges to help free cracked stone.
Clearly, the uppermost cracked stone was totally unusable for tool making, however deeper layers exposed to temperatures of perhaps 200 to 100 degrees, or often less, seemed perfectly suitable for making the relatively small tools found during excavations, and at other Mesolithic sites in the region.
Forms of the fire set surfaces greatly resemble the old ones in the quarry, and waste from the experiments very much resembles what is found in the old waste heaps - crucial indications that fire setting was used at the Melsvik chert quarry 10,000 years ago.
Edited from Past Horizons (November 2013)